No idea why in this case. Unlikely to be due to vortex wake though, as there would have been at least 2 minutes between them at departure and the 747 would have had a pretty decent groundspeed (although very little climb rate probably!). Probably the sector had the 737 on radar headings then to separate against other traffic either inbound or setting up for a more northerly eastbound track? Like I said, just a guess.